and so on. Each number is then used as the base for an alphabet. For example, the word THAT could be enciphered like this:

THAT written as numbers is 20 7 1 20.

I then look at the first 4 numbers on my pad, and add them to my message.

053 + 20 = 73
786 + 7 = 793
423 + 1 = 424
156 + 20 = 176

To complicate things we sometimes use non-carrying addition. We also might place a different letter as the base of the alphabet. So, instead of 053 equaling the first letter, A, it might equal J. Thus, if A was enciphered we would take away 10 to compensate for the J. We use Cyrillic, of course, but this example in your Latin script is probably easier to understand. Because each alphabet is completely random, there is literally no way to break this system (outside of having the codebook and knowing when each pad was used).